It is a known practice to give remote detection missions using microwave radar to satellites in low orbit. Unlike a geostationary satellite, a satellite in low orbit moves relative to a fixed point on the surface of the earth. This movement allows an observation satellite to overfly the earth in a direction that is determined by its orbit. Radar measurements performed at successive moments while the satellite is moving along its path enable a map of the measured parameters to be drawn up as a function of the position of the satellite or other vehicle being used as the radar platform.
A known use of space radar is to measure precipitation rates, or other atmospheric data. A radar wave transmitted at a well-defined polarization is subjected to depolarization as it travels, and also to "backscattering"; by measuring the percentage of reflected power detected by the radar, it is possible to deduce the precipitation rate in the region being probed by the radar. In order to draw up a map of such measurements with adequate three dimensional resolution, it is desirable for the transmitted beam to be very narrow, e.g. to have an annular divergence .beta. lying in the range about 0.18.degree. to about 0.4.degree., so as to probe a relatively precise location with a train of radar pulses. In addition, in order to be able to cover the entire globe in a reasonable length of time, e.g. compatible with the time scale of meteorological changes, it is desirable for the beam to be steerable over a relatively large transverse angle, of the order of .+-.10.degree. to .+-.20.degree. relative to the orbital plane of the satellite.
FIG. 1 is a diagram showing a radar remote detection satellite known in the prior art.
In FIG. 1, a satellite 100 overflies the earth at a height H of about 400 km to about 500 km, with a velocity V.sub.sat which depends on the geometrical parameters of the orbit. The satellite is fitted with equipment appropriate to its mission, such as a solar panel 110 and a radar antenna 101. In the simplest configuration, as it moves past, the radar remote detection satellite illuminates a swath F on the ground, of width G, and occupying a "transverse" illumination plane that contains the nadir and that is perpendicular to the velocity V.sub.sat. The swath F is illuminated by successive microwave pulses whose reflections constitute the radar signal that is received by the same antenna 101 operating in reception, and that is interpreted for the purpose of extracting the desired atmospheric data.
To illuminate the swath F, a fine pencil B of radiation is scanned in the transverse plane using the electronic scanning technique in order to illuminate an approximately rectangular strip on the ground of width G and of length S (in the velocity direction V.sub.sat). The pencil B has angular divergence .beta. (thereby giving a diameter .O slashed. on the ground), and it is radiated at an angle .PSI. that takes a different value on each pulse within a range of values such that the maximum variation .+-..PSI..sub.M is typically of the order of .+-.10.degree. to .+-.20.degree.. Thus, as the satellite moves, successive strips (11, 12, 13, 14, 15, 16, . . . ) are illuminated so as to end up by covering the entire swath F.
By way of example, typical values for the various parameters applicable to remote detection missions already in operation are as follows:
______________________________________ H (km) G (km) S (km) .O slashed. (km) .+-..phi..sub.M ______________________________________ 500 100 3 1.6 .+-.5.7.degree. 430 200 3 2.8 .+-.13.degree. ______________________________________
To improve detection of precipitation rates, it is known that two successive measurements at the same location make it possible to reduce random measurement effects due, for example, to atmospheric disturbances such as gradients or variations in the density or the temperature of the air on the path of the radar wave. It has also been envisaged that two radar measuring equipments could be provided on board a satellite so as to perform two measurements over the same geographical area, with the two measurements being separated by a short period of time. In addition, by looking at the same zone with two different angles of incidence (+.alpha., then -.alpha.) it becomes possible to detect precipitation rates with greater accuracy. Such an installation is known to the person skilled in the art as "stereoradar" by analogy with stereo binocular vision.
This can be obtained using two identical antennas, one aiming slightly in front of the nadir at an angle +.alpha. while the other is aimed slightly behind the nadir, at an angle -.alpha.. A typical value for the angle .alpha. lies in the range about 15.degree. to about 20.degree.. Two strips on the ground are thus illuminated by two respective beams B.sub.F and B.sub.B, these two beams being separated by a distance D=2.times.H.times.tan.alpha., where H is the instantaneous height of the satellite above the ground. Thus, a location illuminated for the first time by the front beam B.sub.F will be illuminated a second time by the back beam B.sub.B after a time interval t=D/V.sub.sat.
A satellite carrying such a payload is known in the prior art and is shown highly diagrammatically in profile in FIG. 2.
As in the preceding FIG., the remote detection satellite 100 includes a solar panel 110 and it moves along a vector V.sub.sat. The payload of this stereoradar satellite comprises two radar antennas each including a reflector (102, 103) illuminated by an array of sources (98, 99) enabling electronic scanning to be performed in the transverse plane. The relative geometry of the sources and of the reflectors is known for the purpose of transmitting a front beam B.sub.F that slopes at an angle +.alpha. relative to the nadir, in the direction of V.sub.sat, and a back beam B.sub.B that slopes at an angle -.alpha. relative to the nadir. The two beams may be emitted either simultaneously or they may be switched on in alternation. Between two transmitted pulses, the equipment operates in reception to receive reflected radar waves in conventional manner.
Several problems arise with the conventional installation of a stereoradar satellite as described above. Firstly a remote detection satellite may have other missions and other on-board equipment, thereby giving rise to problems of space availability on board the satellite: to perform a stereoradar mission in its conventional configuration, it is necessary for both the front face and the rear face of the satellite to be available to carry the reflectors (102, 103). Unfortunately, that is not always the case.
In addition, successive superposition of the "footprints" of the two beams depends on the accuracy with which the two reflectors are positioned and pointed, and also on their relative positioning and pointing. Unfortunately, the accuracy of such positioning is compromised by the complexity of the antennas, in particular by the mechanisms used for deploying them.
FIG. 3 is a diagram showing the same stereoradar remote detection satellite as FIG. 2, but in its launch configuration. It can be seen that for launching, the reflectors 102, 103 must be folded up so as to make it possible for them to be inserted together with the body 100 of the satellite inside the nosecone of the Ariane launcher or inside the hold of the American shuttle, for example. In the simplest case, both reflectors 102 and 103 are hinged to the body 100 of the satellite at hinges 112, 113, and each reflector (102, 103) is made up of a plurality of segments (122, 123; 132, 133) which are likewise hinged about respective hinges (120, 130).
The deploying of such structures in orbit is not without risk, and numerous satellites are handicapped in their missions because of ineffective deployment of vital members such as solar panels, antennas, etc. However, even when deployment is successful, the relative positions of the various hinged portions relative to one another can neither be foreseen, nor estimated, nor controlled with great accuracy, and the greater the number of hinged components, the lower the accuracy possible. Furthermore, the relative positions of those elements can vary over time, e.g. because of differential thermal expansion due to solar heating as the satellite moves.
Finally, such a conventional stereoradar configuration has mass and bulk that are double those of the single beam equipment shown in FIG. 1.